New Docker Image - Javascript port conflict checking

[benyanke]

When the host ports are entered, there really ought to be checking for host port conflicts dynamically, so that you don't need to build the image to find the conflict...

[saarg]

Although not automatically, you do have an overview of all the used ports in the template. 

[MrCrispy]

But its still possible to add a conflicting port. This should be pretty easy to do and show a warning since the list of all ports is already known.

[JonathanM]

Conflicting ports are only bad if you need to run the 2 apps simultaneously. I purposely put krusader and dolphin on the same port, so I can run either one and get whichever file manager I want using the same shortcut. I don't keep either running all the time for security reasons, just start whichever one I need on demand.

[MrCrispy]

That's a good point. But I still think more often than not it would be an error esp for a new user.